#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define endl '\n'
const int N = 205;

int m, n, c;
int a[N];
int b[N];
int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cin >> m >> n >> c;
  rep(i, 1, c) cin >> a[i];
  sort(a + 1, a + c + 1);
  c = unique(a + 1, a + c + 1) - a - 1;
  if (c <= m) {
    cout << c;
    return 0;
  }
  int sum = a[c] + 1 - a[1];
  rep(i, 1, c - 1) b[i] = a[i + 1] - a[i] - 1;
  sort(b + 1, b + c, greater<int>());
  rep(i, 1, m - 1) sum -= b[i];
  cout << sum;
  return 0;
}